A march and rally will be held today in a historic naval city to protest at job losses planned by defence giant BAE Systems.
Unions said they expect a big turnout in Portsmouth following last month's announcement that shipbuilding is to end.
More than 900 jobs will be lost, although Portsmouth will retain repairs and maintenance work.
About 830 jobs will also be lost at yards in Govan and Scotstoun, on the River Clyde in Glasgow, at Rosyth in Fife, and at the firm's Filton office, near Bristol, as a result of a drop in work after the end of aircraft carrier work.
Unite national officer Ian Waddell said: "Portsmouth has a proud 500-year history of shipbuilding. It is a waste and a tragedy that the Government is allowing this legacy to end.
"BAE Systems' workers, their families and the local community are calling on the Government to think again and secure shipbuilding in Portsmouth for another 500 years.
"As the ultimate owner of the Portsmouth dockyards, the Government can generate opportunities to save these valuable shipbuilding jobs and skills."
Gary Cook, GMB regional officer, said: "Not only is the coalition government prepared to throw more than 1,000 highly skilled shipbuilders on the scrap heap, many of whom will be destined for a life on benefits, but they are also playing fast and loose with the national security of these British Isles by having no plan B for warship building.
"Regardless of party politics surely any right-minded individual would not put all their eggs in one basket and would see the real value of retaining a capability at Portsmouth."